Effects

Effects & DSP

Everything runs in f64 end to end — coefficient accuracy and sample precision over CPU efficiency. Audio is captured, converted to f64, pushed through the chain below, and converted back for playback.

Signal flow

capture ─▶ ProcessorChain ─▶ playback

ProcessorChain:
  ├─ APO filter bank      parametric EQ — 14 biquad filter types
  ├─ Convolution          WAV impulse response (partitioned FFT; off by default)
  ├─ Fidelity             FxSound Aural harmonic exciter
  ├─ Ambience             FxSound Lex reverb (Freeverb topology)
  ├─ Surround             FxSound Wide32 mid/side widener
  ├─ Dynamic Boost        loudness maximizer (makeup + lookahead limiter)
  ├─ Bass                 peaking shelf at 90 Hz, ±15 dB
  ├─ Loudness             ISO 226:2023 equal-loudness compensation
  ├─ Crossfeed            Bauer/Meier headphone crossfeed (runs last)
  └─ Dither               TPDF dither to a target bit depth (off by default)

Parametric EQ

• 14 biquad filter types (Audio EQ Cookbook formulas): peaking, low/high shelf, low/high pass, band-pass, notch, all-pass, and more.
• Per-band slope — 12 / 24 / 48 dB/oct (Butterworth cascade on shelves + HP/LP).
• Per-band scope — Stereo | Mid | Side (process the mono sum or the stereo difference independently).
• Dynamic EQ — per-band, level-driven gain morph with threshold / range / attack / release and a band-passed linked sidechain. Zero added latency.
• Linear-phase EQ (optional) — static stereo bands rendered to a symmetric FIR via the partitioned convolution engine; avoids biquad phase rotation at the cost of latency (~171 ms @ 48 kHz). Hybrid: M/S and dynamic bands stay IIR.
• Per-band audition — solo (bypass every other band) and listen (type-aware band-pass of the band's region) to hear what a band is doing.

Convolution / impulse-response loader

Two-stage non-uniform partitioned overlap-save FFT (256-sample head partitions for fixed low latency + 4096-sample tail partitions spread across the cycle). The IR is resampled to the DSP rate (gain-compensated), capped at 10 s; a mono IR broadcasts, multi-channel maps per channel. Load a WAV IR with resonance ir <file.wav>, the GUI Convolution section, or the TUI I picker. Persists in profiles.

FxSound-matched effects

Effect strength matches FxSound so imported .fac presets sound like FxSound. Preset MIDI knob values (Main N, 0–127) map to intensity = MIDI/127, then each effect applies FxSound's own scaling. MUSIC_MODE2 is always active (FxSound v13 forces it), which warps Ambience ×0.34 and Dynamic Boost ×1.8 before their curves.

• Fidelity (Aural exciter) — out = x + 1.5·sin(drive·hp), where hp is a 2nd-order Butterworth high-pass at ~4.5 kHz and drive = intensity · 3.393. Odd-harmonic exciter; the even path is disabled (as in FxSound).
• Ambience (Lex reverb via Freeverb) — knob warped w = intensity·0.34, comb feedback 0.095·10^w (a short room, max ≈ 0.21), wet 0.273 / dry 0.897.
• Surround (Wide32) — full-band mid/side: gain_side = 1 + 3·(intensity·0.7), centre compensation 1 − 0.3·(intensity·0.7). Bipolar (negative narrows toward mono).
• Dynamic Boost — loudness maximizer: makeup gain (≤ +12 dB) then a 0.75 ms lookahead brickwall limiter holding peaks ≤ 0.9.
• Bass — peaking biquad at 90 Hz, Q 2.5, bipolar −15 → +15 dB.

Extra effects

• Loudness — ISO 226:2023 equal-loudness compensation (the "loudness" button most consumer EQs have).
• Crossfeed — Bauer/Meier headphone crossfeed (one-pole low-pass ~700 Hz on the opposite channel, mono-level compensated). Runs last of the effects.
• Dither — final-stage TPDF dither to a target bit depth (off by default).